The present invention relates generally to a permanent magnet type eddy current braking system, and more specifically, to a permanent magnet type eddy current braking system capable of providing a partial braking condition.
In the permanent magnet type eddy current reduction apparatus disclosed in Japanese Patent Laid-Open No. Hei 4-88,867 and the like, a number of permanent magnets (hereinafter merely referred to as magnets) are juxtaposed on the outer peripheral surface of a single magnet support tube which is arranged interiorly of a rotating brake drum so that polarities are N, N, S, S, . . . in a circumferential direction, and a guide tube having a number of ferromagnetic plates (pole pieces) is arranged between the brake drum and the magnets. When a pair of magnets different in polarity are opposed to the inner surface of a common ferromagnetic plate by rotation of the magnetic support tube, a magnetic field from the magnets is interrupted inside the ferromagnetic plate and is not applied to the brake drum, producing a non-braking state. Conversely, when a pair of magnets having the same polarity are opposed to the inner surface of a common ferromagnetic plate, a magnetic field from the magnets passes through the ferromagnetic plate and is applied to the brake drum, producing a braking state in which an eddy current flows into the rotating brake drum. A partial braking force may be obtained by holding a magnetic support tube in an intermediate position between a braking position and a non-braking position by an air pressure actuator.
In the aforementioned apparatus, even if a diameter of the brake drum is not enlarged, an axial dimension can be lengthened whereby the braking force can be increased. It is therefore advantageous in the case where a mounting space of the eddy current reduction apparatus has allowable axial space. However, the axial dimension of the entire magnet support tube becomes longer and a great driving force is necessary for driving the entire magnet support tube to obtain the braking, partial braking and non-braking states. Therefore, the dimension and capacity of the air pressure actuator increases, the mounting property relative to the vehicle is poor, consumption of pressure air increases, and the number or dimension of magnets arranged on the magnet support tube is reduced.
The eddy current reduction apparatus disclosed in Japanese Patent Laid-Open No. Hei 4-12,659 addresses the aforementioned problems. However, diametrical space of the brake drum is limited, and if an attempt is made to lengthen the axial dimension of the magnets to secure a greater braking force, the ferromagnetic plate has to be increased in thickness in order to confine an axial short-circuiting magnetic circuit during a non-braking state and thereby prevent leakage of magnetism to the brake drum.
In view of the aforementioned problems, the object of the present invention is to provide an improved permanent magnet type eddy current braking system in which even with relatively thin ferromagnetic plates, leakage of magnetism during non-braking conditions is confined.